1. Field of the Invention
The present invention relates to a fuel control method and apparatus for an internal combustion engine and, more particularly, to control method and a fuel apparatus which achieves increased accuracy by taking into account the effect of condensation of fuel on the internal walls of the fuel intake flow passage.
2. Description of the Prior Art
In fuel control systems for internal combustion engines, the accumulation of a fuel film on the internal surfaces of the fuel intake passages due to condensation introduces a source of error which causes inaccuracy in fuel control. That is, the condensation of fuel, which adheres or attaches to the walls of such flow passages, causes the actual rate of fuel intake to differ from the fuel injection rate at the fuel injectors. Several prior art technologies have been developed to compensate for this difference, such as the following.
Japanese Patent Publication No. 62-48053 (1987) discloses a compensation arrangement comprising a device for calculating a value for the current equilibrium intake surface fuel (that is, the amount of fuel contained in the fuel film which adheres to the walls of the fuel passages) as a function of engine operating parameters, a device for calculating a current intake surface fuel time constant as a function of engine operating parameters, a device for calculating current actual intake surface fuel as a first order differential function of the previous actual intake surface fuel and a transition rate of the previous actual intake surface fuel, and a device for calculating the current transition rate of intake surface fuel as a function of the current equilibrium current intake surface fuel, the intake surface time constant and the current actual intake surface fuel, and iteratively calculates the transition rate of intake surface fuel to determine a fuel demand in conjunction with a required fuel flow rate. In the Detailed Description of the Invention in the Japanese Patent Publication, there is this following description, "an abrupt acceleration causes an increase in the rate of fuel accumulation on the wall of the intake flow passage and an abrupt deceleration causes a decrease in the rate of fuel accumulation on the wall of the intake flow passage. This is lead from the change in vapor pressure. The higher the vapor pressure is, the more the fuel accumulates on the wall of the intake flow passage. The vapor pressure is a partial pressure, the pressure inside the intake flow passage is, therefore, affected mainly by air." Therefore, the greater the air suction flow rate is, the greater the increase in the amount of liquid film in the intake pipe. According to the disclosure in this Japanese Patent Publication, the equilibrium intake surface fuel relates to the absolute pressure in the intake manifold, and is closely related to the engine load. Therefore, when the absolute pressure in intake manifold is represented on the abscissa and the equilibrium intake surface fuel is represented on the ordinate, a family of curves, depending on the rotating speed of engine is generated. As an embodiment, the absolute pressure in the intake manifold and the rotating speed of the engine are used in calculating the current equilibrium intake surface fuel and as the parameters for the current intake surface time constant.
Japanese Patent Publication No. 3-59255 (1991) is similar to Japanese Patent Publication No. 62-48053 (1987). In JP 3-59255, a wall surface fuel condensation and evaporation rates are determined based on engine operating parameters, including at least the pressure in the intake manifold. An increase or amount decrease in the amount of wall surface fuel during a given cycle period is calculated and accumulated based on the condensation and evaporation rates and the results are used to correct the wall surface fuel and, finally the basic fuel injection rate.
The wall surface fuel condensation and evaporation rates are functions of the pressure in the intake manifold, the temperature of engine water, the rotating speed of engine and the intake air flow velocity. The higher the pressure in intake manifold is, the greater the wall surface fuel condensation rate is. In other words, air flow rate increases, the larger the wall surface fuel condensation rate increases with increasing air flow rate.
According to the prior art described above, the fuel condensation rate is a function of (that is, proportional to) the suction pressure. (The condensation rate increases as the suction pressure approaches atmospheric pressure). Therefore, there is a disadvantage that when the intake pressure approaches atmospheric pressure, such as in low speed high load operation, a pulsation occurs in intake air flow, which finally decreases the accuracy in the fuel injection rate.
Further, according to the prior art described above, since the calculations for the fuel condensation rate and the fuel evaporating rate are performed by inputting two variables (intake manifold pressure and engine speed), the calculating load borne by a processing unit becomes large because of the memory area necessary to store condensation and evaporating rates in advance, the retrieving time for these rates is increased, and the matching work process increases because of the large number of variables to be stored in advance in connection with the above problem.